The production of filler-containing silicone compositions, i.e. the production of liquid-phase homogeneous compositions which are composed essentially of finely divided solids and polyorganosiloxanes, is an important process which is widely used. For example, crosslinkable silicone elastomer compositions such as the compositions known to those skilled in the art under the designations HTV, LSR, RTV-2 and RTV-1, silicone master batches, silicone pastes, etc., are all produced by such a process.
Continuous and discontinuous (batchwise) processes for producing filler-containing silicone compositions are considered distinct processes by those skilled in the art. In batch processes, all constituents of the filler-containing silicone composition are, in a first step, introduced into a mixing apparatus, e.g. a mechanical mixer, if desired in portions, and then mixed with one another. The finished silicone composition is subsequently, in a second step, removed from the mixing apparatus before the mixing apparatus is again charged with starting materials for a further batch.
In continuous processes, the starting materials, in the ratio corresponding to the formulation desired, are fed continuously into a mixing apparatus, e.g. a reciprocating kneader, mixed and if appropriate, degassed, and the finished silicone composition is simultaneously removed continuously from the mixing apparatus, as a result of which a dynamic equilibrium between starting materials introduced and silicone composition discharged is established. Such a process for the continuous production of HTV silicone compositions is described, for example, in EP-A-570387.
Continuous processes for producing filler-containing silicone compositions represent a tremendous step forward compared to the batch processes in respect of economics (significantly higher space-time yields) and product quality (lower fluctuations).
In the production of silicone compositions which have a high content of actively reinforcing fillers, there is a fundamental difficulty of dispersing relatively high proportions of a finely divided solid in liquid polyorganosiloxane. Due to the high specific surface area of actively reinforcing fillers and the associated strong interaction between filler and polyorganosiloxane, phase inversion (crumbling) occurs rather easily. Phase inversion, in this sense, means a disintegration of the previously compact, liquid polyorganosiloxane phase into a pulverulent, solid-phase material. The further processing of the pulverulent composition to give a compact, homogeneous, liquid-phase silicone composition requires phase reinversion (compacting) which is difficult to bring about, and results in numerous disadvantages.
To achieve compacting of the pulverulent composition, the mixing apparatus has to be configured such that the pulverulent composition is banked up and compacted to an extreme degree in order to be able to introduce sufficient shear energy to achieve compacting. In terms of the apparatus structure, this can be done by installing constricting rings which reduce the cross section of the (tubular) mixing apparatus. The high compacting pressure to which the pulverulent composition needs to be subjected, and the simultaneous shear, are both associated with very high dry friction between the particles of the pulverulent composition. This results in mechanical destruction of the high molecular weight polyorganosiloxanes, which causes an adverse change in the final properties of the finished products produced from these silicone compositions. In particular, it is found that highly filled HTV in particular silicone compositions whose continuous production has involved a phase inversion display a greater tendency to undergo crepe hardening and to become brittle during storage than do HTV silicone compositions which are made up of the same constituents but have been produced batchwise and without phase inversion, e.g. in a kneader.
The intensive shear under high compacting pressure also leads to increased equipment wear, as a result of which abraded metal contaminates the silicone composition.
The difficulty of compacting pulverulent silicone compositions in continuous mixing apparatuses is also shown by the fact that even pressing the composition under high pressure with the simultaneous application of shear sometimes compacts the compositions only incompletely, so that when the pressing pressure is released while continuing to apply shear, non-cohesive silicone composition particles are formed again. The formation of a stable, compact, liquidphase silicone composition therefore frequently occurs only after a plurality of compaction steps. This also means that actual kneading in the sense of a laminar and dispersive mixing processes only occurs in the downstream part of the mixing apparatus, shortly before discharge of the composition; the operating window of the process is considerably narrowed thereby, i.e. slight changes in the apparatus, raw materials, process parameters (throughput, temperature, vacuum, etc.) can result in serious changes in the product quality.
All known continuous processes for producing highly filled, highly viscous silicone compositions such as HTV silicone compositions suffer from these disadvantages. In general, the problems associated with phase inversion are all increasingly more serious with higher filler content, higher specific surface area of the filler, and increased filler-polyorganosiloxane interaction.
EP-A-849 331 describes a process for producing highly filled silicone polymer/solid premixes which avoids the disintegration of the silicone composition during mixing, by addition of volatile, low molecular weight organosilicon compounds which have no hydrolyzable groups. After the incorporation of the solid, the low molecular weight organosilicon compounds are removed again. This process is not very suitable for the continuous production of highly filled silicone compositions since the low molecular weight organosilicon compounds have to be mixed into the polyorganosiloxane in a very large amount to prevent phase inversion; in addition, the subsequent removal of large amounts of low molecular weight organosilicon compounds is costly and presents safety problems.